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Objective: To determine the incidence of and risk factors for exercise-associated hyponatremia (EAH) in cyclists completing a long-distance bike ride and to assess whether postexercise serum NT-proBNP concentration (brain natriuretic protein precursor) differed between riders with and without EAH.

Conclusions: Exercise-associated hyponatremia was found to occur in 4.5% of the study group and was associated with weight gain during a prolonged bike ride. Postride [Na+] varied inversely with weight change and with fluid intake. Three of 4 hyponatremic riders had significant elevations of [NT-proBNP]. These results support the hypothesis that overconsumption of hypotonic fluids in this setting is the most important cause of EAH.

*MP Sports Physicians, Victoria, Australia

†Sports Medicine Practice, Hobart, Tasmania, Australia

‡Chemical Pathology, Melbourne Pathology, Melbourne, Australia

§Centre for Health, Exercise and Sports Medicine, University of Melbourne, Melbourne, Australia

¶Australian Centre for Research into Sports Injury and Its Prevention, Monash University, Clayton, Australia.

G. Harris has received payment from Bicycle Victoria for authorship of magazine articles on topics unrelated to this research.

K. Sikaris is a salaried chemical pathologist with Melbourne Pathology. Melbourne Pathology provided laboratory analysis for this research free of charge.

P. McCrory has received grants, royalties, and travel expenses from bodies unrelated to this current research, both directly and through the Centre for Health, Exercise and Sports Medicine. Neither he nor CHESM have received any payment or support in kind for any aspect of the submitted work.